Spiral Arms

Looking from above the disc of a spiral galaxy (like the Milky Way
where we live) a new picture appears. We can see dense spiral arms in the disc
of the Galaxy. We think it is gravity which created the spiral pattern,
although the details are not clear. The spiral arms are places where the gas of the disc is squashed together.

Note that we can never see our own Galaxy
from outside, since we are embedded within it and it would take millions of
years to travel out of it. Thus we are not sure what our Galaxy looks like, but
we think it is probably a typical barred spiral galaxy, so-called
because it has a small bar across the center, perhaps similar to the one shown
in the next image.

Image of barred spiral galaxy NGC 6217
from NASA, ESA, and the Hubble SM4 ERO Team

The disc of our Galaxy measures about
100,000 light-years across and 1000 light-years thick.